Modern phased array systems are required to operate over wide frequency bandwidths with a single radiating aperture. In such broad band environments, processing functions that have previously been performed by individual antennas, need now to be performed by a single phased array.
A critical parameter of many RF signals is their polarization, requiring an array to respond to any linear, circular or elliptical polarization. This in the art is known as polarization diversity or polarization agility. Antenna polarization agility may be achieved with orthogonally disposed pairs of radiating elements that are electronically processed via a vector controller. Such a vector controller is described by Mohuchy in U.S. Pat. No. 5,933,108, issued Aug. 3, 1999 and is entitled “Gallium Arsenide Based Vector Controller for Microwave Circuits”, the disclosure of which is incorporated herein by reference. Polarization agility of a phased array is in much demand.
Significant advances in broadband solid-state power generation has also placed a new emphasis on phased arrays to efficiently combine the power of individual devices into high-power transmissions by exploiting a magnification property known as the “array factor”. Commensurate with this trend, demands for high transmitted effective radiated power (ERP) have increased. In addition, operating frequency range has been lowered into the HF/VHF region.
Dimensions of an antenna are inversely proportional to its operating frequency and wavelength, typically measured in tens of feet at HF/VHF frequencies. Consequently, size and weight of low-frequency antenna systems are of concern. This concern is particularly acute in mobile installations on aircraft, ground vehicles and even ships. To circumvent these limitations, shortened and inefficient antennas have been developed, which produced undesirable radiation performance and caused significant secondary inefficiencies in power and heat generation. More efficient radiators have been developed, such as Log-Periodic or Yagi arrays, which require considerable volume and are useable with the so called “big-bottle” transmitters. These radiators have limitations in broadband array applications, due to their element size that is incompatible with grating lobe suppression.
Conventional efforts on size reduction primarily addresses the microwave frequency region. A representative effort is disclosed by Wang et al. in U.S. Pat. No. 5,589,842, issued Dec. 31, 1996, which is entitled “Compact Microstrip Antenna with Magnetic Substrate”. Wang et al. disclose different planar radiators which typically are cavity backed for deployment on metallic surfaces. Although Wang et al. deal with arraying elements in circular disposition, they do not deal with any limitations brought about by grating lobe issues.
Dempsey et al. in U.S. Pat. No. 5,563,616, issued Oct. 8, 1996, which is entitled “Antenna Design using a High Index Low Loss Material”, disclose a high index of refraction medium having high matched values of relative permeability and relative permittivity. The Dempsey et al. approach favors the VHF frequency region. Applying their approach at HF frequencies, however, when deploying a polarization-diverse phased array, results in an antenna depth that far exceeds the space availability on most mobile platforms.
A need exists to drastically reduce the size of HF/VHF radiating elements, both in surface area and in depth. A need also exists to further decrease the element surface area of an array, while improving impedance characteristics of the array. Yet another need exists to improve the polarization capability of the array at HF/VHF frequencies. The present invention addresses these needs.